WordPress add page under admin submenu and retaining the active status of the parent submenu page in the menu

I have added a page in the admin menu(pxmag-menu) and a submenu(pxmag-plans). There is another page(pxmag-plans-edit) set under the submenu(pxmag-plans) as the parent page.

public function __construct() {     require('pxMagAdminPlans.class.php');     $  this->admPlanObj= new pxMagAdminPlans();      add_action('admin_menu', array($  this, 'add_plan_admin_menu')); }  public function add_plan_admin_menu() {     add_menu_page(__('Dashboard', 'textdomain'), get_bloginfo('name'), 'manage_options', 'pxmag-menu', array($  this, 'pxmag_dash'), 'dashicons-welcome-view-site', 6);              add_submenu_page('pxmag-menu', __('Subscription Plans', 'textdomain'), 'Plans', 'manage_options', 'pxmag-plans', array($  this->admPlanObj, 'plan_admin_menu_page'));     add_submenu_page('pxmag-plans', __('Add/edit Plans', 'textdomain'), 'Add/edit plans', 'manage_options', 'pxmag-plans-edit', array($  this->admPlanObj, 'plan_admin_menu_edit')); } 

All the menu and submenu pages load fine.

But, when I open this page(pxmag-plans-edit), the menu selection in the WordPress admin shows nothing as current item, whereas the pxmag-plans is supposed to be the current selection.

(It is supposed to work like: when I click ‘Posts > Categories‘ and subsequently open the ‘edit category‘ page, the ‘Posts > Categories‘ option in menu keeps selected).

What is going wrong? What is the correct process?